1. Field of the Invention
The present invention relates to a method for forming color images, and, more particularly, to a method for forming color images which comprises processing a photographic element containing imagewise distributed silver in the presence of a complexing agent, an oxidizing agent and a dye to thereby oxidatively bleach the dye.
2. Description of the Prior Art
A general method for forming color images is a method for obtaining an azomethine or indoaniline dye which comprises processing a silver halide light-sensitive material with an aromatic primary amine developing agent in the presence of one or more couplers. This color development technique using silver halide is based upon the method invented by L. D. Mannes & L. Godowsky, which has since then involved a variety of improvements and is now universally adopted in this art.
The color development method using an aromatic primary amine developing agent involves the following disadvantages:
(1) The dye formed shows poor light fastness, heat resistance and humidity resistance and the color images suffer marked discoloration with the passage of time.
(2) The aromatic primary amine developing agent is hazardous to the skin, e.g., causing skin poisoning, and, for this reason, the developer must be carefully used.
(3) Since dye images and the oxidation product of the color developing agent must be present in equivalent amounts, it is theoretically impossible to reduce the silver halide participating in the formation of the dye images to an amount less than the amount stoichometrically required. Known techniques for reducing the silver halide amount include a method for reducing silver halide that is present in an excess amount than that stoichiometrically required amount as must as possible, on one hand, and on the other hand, a method for reducing the amount stoichiometrically required per se. With regard to the latter technique, for example, 2-equivalent couplers have been developed which can form one mol of a dye by two mols of silver halide. However, even with this technique, it is not possible to reduce the amount of the silver halide in the light-sensitive material to an amount less than 1/2 that of the light-sensitive material containing the 2-equivalent couplers.
A color photographic process other than the above-mentioned processes employed at present is based on a silver-dye-bleach photographic process. This process is also based on the color photographic process disclosed in U.S. Pat. No. 2,270,118 and, since zero dyes are used in the color process, the color images formed by the process generally have excellent light resistance, heat resistance and moisture resistance.
A typical photographic element used for the silver-dye-bleach color photography has three silver halide photographic emulsion layers respectively sensitized to red, green, and blue light, and having associated therewith, respectively, and bleachable cyan, magenta and yellow dye. Such a photographic element provides color photographic positive images through the following processing:
(1) The photographic element is image-wise exposed.
(2) The exposed photographic element is developed in a silver halide developer to form negative silver images, the photographic element is then processed in a dye bleach bath which oxidizes the silver images to a silver salt and concurrently decolorizes the associated dye pattern, and, finally the photographic element is fixed and washed to remove the residual silver salt, whereby dye images are obtained which are photographically the reverse of the initial silver images. The silver-dye-bleach process is generally described in, for example, U.S. Pat. Nos. 3,498,787 and 3,503,741, Canadian Pat. No. 790,533 and A. Meyer, "Some Features of the Silver-Dye Bleach Process", The Journal of Photographic Science, Vol. 13, 90-97 (1965).
In the silver dye bleach process as described in U.S. Pat. No. 2,270,118, dye images are formed by processing dye-containing layers having silver images with an acid solution which decomposes the dyes at the silver-containing areas. The decomposition or destruction of the dye is accelerated by various "catalysts" such as phenazine. Also, the reaction in these dye bleach systems is considered to proceed on a stiochiometric basis (for example, it is suggested that 4 atoms of silver are required for decomposing one azo dye group in Column 1, lines 18-21 of U.S. Pat. No. 3,340,060).
However, these silver dye bleach processes have the following disadvantages:
(1) Since a large amount of silver is required for bleaching the dyes, the photosensitive materials must contain a large amount of silver halide in the silver halide photographic emulsion layers.
(2) Since a strongly acidic processing solution which is highly corrosive is usually used in these processes, difficulties are encountered in preserving and handling the solution.
Recently, numerous investigations have been made for saving silver as a resource, increasing the efficiency of the reaction system, and improving the quality of the color images formed by reducing the amount of silver required to decompose each molecule of dye.
Several patents are known which deal with various types of silver dye bleach image forming processes, for example, U.S. Pat. No. 3,716,362 Meier, U.S. Pat. No. 3,259,497 Wartburg and U.S. Pat. No. 2,564,238 Sprung. These patents are discussed below and compared to the present invention, disclosure relative to the present invention not being part of the prior art, of course, but being offered to offer a valid comparison to the prior art. Reference should also be made to later discussed FIG. 2 for a complete understanding of the subject matter involved.
Turning to U.S. Pat. No. 3,716,362, this patent teaches a process at column 1 lines 64 to 65 wherein metallic silver is removed from a photographic material without decomposition of a dye. That is, the reaction of Ag.degree..fwdarw.Ag.sup.+ L.sub.2 occurs in this process as shown in FIG. 2. Further, the reaction of Meier involves materials which exhibit the following relationship between their complex forming constant and oxidation-reduction potential: EQU E.degree.(AgL.sub.2.sup..sym. +e.sup..crclbar. .revreaction.Ag.degree.+2L)=E.degree.(Ag.sup..sym. .revreaction.Ag.degree.)-(RT/F) lnK.sub.1
where R, F and T denote gas constant, Faraday constant and temperature respectively. When a complexing agent is present, E.degree.(Ag.sup.+ .fwdarw.Ag.degree.) is constant and, therefore, the oxidation-reduction potential of Ag can be represented by the complex forming constant K.sub.1.
On the other hand, the present invention teaches a process in which a dye is decomposed at the areas where silver is present, i.e., metallic silver is oxidized by a peroxo sulfate into the Ag(I)-polymer complex (hereinafter, "Ag(I)-polymer complex" is referred to "silver complex") and the Ag(I)-polymer complex is further oxidized by the peroxo sulfate into the Ag(II)-polymercomplex. In the reaction the complexing agent acts to reduce the oxidation-reduction potential of silver (i.e., Ag.sup..sym. .fwdarw.Ag.sup.2.sym.); thus, this process can be schematically illustrated as follows: ##EQU1## where K.sub.2 represents the complex forming constant with Ag.sup.2.sym., K.sub.1 represents the complex forming constant with Ag.sup..sym., and Py represents the pyridine unit of the polymer. As is apparent from the above schematic, the complex forming constants directly influence the reaction in the form K.sub.1 /K.sub.2 and, therefore, E.degree. cannot be expressed by K.sub.1 as in Meier.
The process of this invention differs from the process of Meier as follows:
Meier's process: ##EQU2##